Interactions between domatia-dwelling ants & Acacia erioloba in Namibian savanna

1. Interactions between domatia-dwelling ants & Acacia erioloba in Namibian savanna Heather Campbell, Mark Fellowes & James Cook School of Biological Sciences, University of Reading, Reading, RG6 6AS

2. Ants and Acacias Ant–plant associations are widespread (in over 100 genera of tropical angiosperms and at least 20 plant families and in five of 12 ant subfamilies) Research focus on 3 plant genera: Acacia; Cecropia; and Macaranga • Acacia benefits: • Reduced encroachment • – competition & fire break • Pollination • Protection against herbivores • Ant benefits: • Shelter • Food - extrafloral nectar, food bodies, third party insects i.e. aphids, scale insects

3. Study site Kuzikus is approximately 180km south east of Windhoek, situated in the Namibian Central Kalahari. The study habitat is Acacia-dominated savannah; however the site also incorporates dunes, saltpans, bushveldt and grassland. Acacia-dominated savanna, Kuzikus Wildlife Reserve, Namibian Central Kalahari

4. Species Pollinators Sap sucking insects Acacia erioloba Thorn-dwelling ants Tapinoma Herbivores Insects e.g. Gonometa postica caterpillars Cataulacus Mammals Crematogaster Tetraponera

5. Questions Regarding herbivory on A. erioloba we tested: 1. Do patrolling worker ants act as a deterrent to insect herbivores? 2. How important are G. postica larvae in the defoliation of trees? 3. Which ant species are responsible for antiherbivore activity?

6. Methods • 32 trees sampled (16 trees per trial) – two adjacent branches selected; ants excluded from experimental branch and control left with natural ant fauna • After 11 days branches removed with secateurs and sealed in plastic bags – this captured any herbivores present on the branch • Estimates of leaf damage on each branch made by recording the number of leaflets with herbivore damage out of the total number of leaflets (for 15 leaves per branch) • Herbivores collected from branches were counted and identified • Experiment repeated twice – before and after hatching of • Gonometa silk moth larvae

7. Results • 1. Do patrolling worker ants act as a deterrent to insect herbivores? Branches without patrolling ants showed herbivore damage to 22% of leaflets, whereas branches with ants had only 9% leaflet damage (t32 = -6.83, p < 0.001). The mean (+SE) percentage of leaflets damaged per branch, according to treatment, for the first and second trials of the ant exclusion experiment. First trial; damage was 13% on ant excluded branches, nearly double (7%) the damage on branches with patrolling ants (t16= -3.52, p<0.005). Second trial ; branches without ants had damage to 33% of their leaves as opposed to 11% damage in branches with ants (t16=-6.73, p<0.001). N = 16 trees for both trials. Error bars show standard errors.

8. Results • 1. Do patrolling worker ants act as a deterrent to insect herbivores? Herbivorous insects were found in greater numbers on branches without ants than on those with ants (t32 = -3.87, p < 0.001), although the relationship was more pronounced in the second experiment than in the first (first experiment; t16 = -2.42, p < 0.05; second experiment t16 = -3.04, p < 0.01). The mean (+SE) number of insects per branch, according to treatment, for the first and second trials collected at the end of the ant exclusion experiment. N = 16 trees for both trials. Error bars show standard errors.

9. Results • 2. How important are G. postica larvae in the defoliation of A. erioloba trees? The number of lepidopteran larvae are not affected by the presence or absence of patrolling ants (t32 = 0.1817, p = 0.8564). There was no increase in lepidopteran numbers between trials on control or experimental branches (control: t16= 1.57, ns, experimental: t16=1.31, ns). So differences in levels of herbivory cannot be attributed to an increase in presence of Gonometa larvae. The number of hemipterans and coleopterans are significantly lower on ant patrolled branches than branches without ants (Hemiptera: t32 = -3.33, p < 0.005, Coleoptera: t32= 2.31, p<0.05).

10. Results 3. Which ant species on A. erioloba are responsible for antiherbivore activity? Analysis showed no significant association between herbivore damage to leaflets and different ant species present on branches (ANOVA, n=32, d.f.=4, ns). Behavioural experiments suggest only Crematogaster behave aggressively, which supports data The average percentage of leaflets damaged per branch, for branches with patrolling ants, according to ant species present. The category of multiple ants represents any combination of ant species where more than one species was present. Damage was recorded as number of herbivore damaged leaflets out of total number of leaflets per pinna. Pinna data was pooled for all leaves measured for damage on each branch, giving an average of leaflet damage per branch. N = 16 trees.

11. Conclusion Ants DO defend A. erioloba but are mainly effective against hemipteran herbivores Further research needed to identify which species are responsible for defence Tapinoma Crematogaster Cataulacus Tetraponera

12. Acknowledgements • BBSRC for funding my PhD • Ministry of Environment and Tourism, Namibia for permission to carry out this research (permit # 1614/2011) • My supervisors Dr Mark Fellowes and Professor James Cook • My field assistant Ian Townsend • Staff at Kuzikus for logistical support • Dr Graham Stone, University of Edinburgh for advice and loan of equipment • Brian Taylor for advice on ant identification